4,4{40 -Diaminostilbene-2,2{40 -disulfonic acid derivatives and processes using same

ABSTRACT

Novel bistriazinylaminostilbene derivatives of the general formula:   wherein X and Y, which may be the same or different atoms or groups, are chlorine atoms, hydrogen atoms, alkyl groups such as methyl, ethyl, n-butyl, ter.-butyl and n-octyl groups or methoxy groups and M is a cation which is hydrogen, alkali or alkaline earth metal or ammonium cation or an ammonium cation derived from a primary, secondary or tertiary amine. The derivatives are useful as fluorescent brighteners for paper or polyamide fibers and in effecting simultaneous resin finishing and fluorescent brightening of cellulosic fibers.

United States Patent Yoshikawa et al.

4,4 '-DIAMINO STIL:BENE-2,2 '-DISULFONIC ACID DERIVATIVES AND PROCESSES USING SAME Inventors: Motohiko Yoshikawa, Omiya;

Yoshiaki Shimizu, Kawaguchi; Yoshiharu Tanaka, Tokyo; Kazuo Ishii, Yono, all of Japan Nippon Kayaku Kabushiki Kaisha, Tokyo, Japan Filed: Jan. 15, 1973 Appl. No.: 323,431

Assignee:

Foreign Application Priority Data Aug. 4, 1972 Japan 47-78139 US. Cl 260/240 B; 106/176; 117/335 R;

117/33.5 T; 162/162; 252/301.2 W Int. Cl. C07D 251/14; C0913 23/00 Field of Search 260/240 B References Cited UNITED STATES PATENTS 7/1960 Carroll et a1. 260/240 B FOREIGN PATENTS OR APPLICATIONS 528,443 7/1956 Canada 260/240 B 1,100,584 3/1961 Germany 260/240 B 1,965,585 7/1971 Germany 260/240 B Primary Examiner-John D. Randolph Attorney, Agent, or Firm-Lane, Aitken, Dunner & Ziems [57] ABSTRACT Novel bistriazinylaminostilbene derivatives of the general formula:

wherein X and Y, which may be the same or different atoms or groups, are chlorine atoms, hydrogen atoms, alkyl groups such as methyl, ethyl, n-butyl, ter.-butyl and n-octyl groups or methoxy groups and M is a cation which is hydrogen, alkali or alkaline earth metal or ammonium cation or an ammonium cation derived from a primary, secondary or tertiary amine. The derivatives are useful as fluorescent brighteners for paper or polyamide fibers and in effecting simultaneous resin finishing and fluorescent brightening of cellulosic fibers.

16 Claims, No Drawings 1 2 4,4DIAMINOSTIIJBENEQQ'-DISULFONIC ACID and fiber and find a wide range of useful applications. DERIVATIVESAND PROCESSES USING SAME In particular, the compounds of formula are very stable in acid media i.e. do not readily precipitate, and can DETAILED DESCRIPTION OF THE INVENTION be used together with alum or resin sized in paper mani h i i 5 ufaetur'ingprocesses orwith casein-clay coatings on pa- This invention relates generally to novel bisper. Furthermore, the compounds of the present inventriazinylaminostilbene derivatives represented by the tion offer the advantage that when used in a resin treatfollowing formula l ment bath, fluorescent brightening of the fibers can be 0,,M SO M $0 M I $0 M wherein X and Y, which be the same or different affected Simultaneously with the resin finishing P atoms groups, are Chloride atoms hydrogen atoms, cess. They impart excellent brightness and light resisalkyl groups such asme'thyl, ethyl, n-butyl, ter.'-butyl tance 10 nylon: treated therewithand y grolfps or P Xg F P and M is f The use of the compounds of the invention is degen atom, alkali or alkal ne earth metal or ammonium scribed in greater detail below cation or an ammonium cation derived froma primary, secondary or tertiary amine. These derivatives have ppl n to p p (SiZe W 1 pigmen o ing now been found to be useful for'fluorescent brightening process) of paper or polyamide fibers and for' effecting simultaneous resin finishing and fluorescent brightening of cel- 3 T Compounds f f l 1 )v are i l f use i IUIOSIC fibCI'S '18 use Of a resin the so pray ize press o pigment oating pr i derivatives present therein- I I cess wherein paper, e.g. sized paper, is brightened after h derivatives reprfesehted y the formula y 1 being formed-into a sheet, for example during or after he Prepared y Condensing about 2 moles of Sodium the step of drying. In the most widely used prior art aniline-2,5-disulfonate with about 2 moles of cyanurie 40 ,method f brightening paper, the internal addition chloride at 10C'or below and then condensing the recess, fl s t brightenefg are dd d to th paper suhh'lg Pmduct first with about 1 Thole of Sodium stock in a. beating or mixing chest. Recently, however, diamih5mbehe'2-2I'disulfohate at or below'ahd the internal addition process has become less frethen about 2 moles Of phenol OI a SUbStItutd dquently used for fluorescent b ighteners due to ri ati thereof at to Phenol 0r P eieneies with respect to the time required for treating, fivatives which y he d h aforementioned P -the-efficient use of brighteners and the disposal of Parath/e Process are rcprerslehted h fhhowlhgforr- Waste liquor problem. On the other hand, brightening mula i e by surface treatment or surface application has become x morefrequently used. Most of the fluorescent brighteners which are presently used in the surface treating or H Q I I surface application processes are brighteners of the i Y I i H type that have been used in 'theprior artinternal addition processes. With the use of such priorart fluoreswherein and Y have the same meanings as defined ,ce nt brighteners in the surface treating processes. the for the formula l Examples of such compounds indesired brightness is obtained only with difficulty. clude the following: phenol, o-chlor'o phe nol, Moreover, the material so treated shows fluctuations in chlorophenol, p-chlor ophenol, 2,4-dichlorophenoh' 'b'rightn'e'ss. Accordingly. thereexists a need in the art methoxyphenol, o-cresol, p-cresol, m-cresol, '2-n i ethylfor agents with high brightening properties suitable for 4-chlorophenol, 2,4-xylenol, 2,6-xylenol, 3,4-xylenol, "113C in surface treating or surface application processes. 2,5-xylenol, p-ter.-butylphenol, p-n' butylpheiiol' and It hasnow been discovered that the compounds of forp-n-octylphenol. I g,mula (l) possess excellent properties as fluorescent The compounds of formula ll) thus obtained (prefbrightenerswhen used in surface treating or surface aperably alkali metal salts such as sodiurrf'and potassium plication. processes. In concentrations where convensalts, and mono-, diand-' -trialkanolamine salts), in the tional fluorescent brighteners (for example the comform of powder'per se or'in the form ofia liquid solupound offormulaflA) below) show. a quenching phetion, are excellent brightening agents for'high-molecunomenon (ie yellowing phenomenon brought about lar weight organic materials such as' textile materials by concentration quenching), for example 0.6 percent for a size press process; 2 percent, based on mineral pigment, for a coating process, the compounds of the present invention (formula 1) don't produce such a phenomenon. As a consequence, high brightness is easily obtainable which has not been attained with any 5 prior art fluorescent brighteners (for example the compound of formula (A) shown below).

SO Na sO Na SO Na Furthermore, the compounds of the present invention (formula 1) are very compatible with a variety of pigments of mineral origin and with a variety of binders such as polyvinyl alcohol, latex, starch and casein.

They also have excellent compatibility with optionally 25 employed in the standard size press process or pigment O coating process, binders or mineral pigments of any type may be used. Examples of suitable binders include casein, oxidized starch, enzymatically modified starch and other starch derivatives, soybean protein, polyvinyl I alcohol, latices, acryl resins, glue, carboxymethyl cellu- 4 lose and the like. Examples of suitable mineral pigments include clays, agalmotolite, calcium carbonate, aluminum hydroxide, titanium oxide, zinc oxide, satin white, calcium silicate, silica and the like.

The compounds of the present invention also exhibit 50 good brightening properties in starch/latex systems which have recently become widely used. Even where inexpensive clay with inadequate whiteness is used instead of expensive pure white clay, adequate brightness can be attained through the use of somewhat increased 55 concentrationof the compounds of this invention.

ii.' Application for simultaneous resin treatment and fluorescent brightening in the same bath Methods for effecting resin finishing and fluorescent 6O brightening of cellulosic fibers and like materials may fall into two categories: the first being the type which I is most conventionally employed, wherein the resin treatment is carried out after fluorescent brightening;

and the other wherein fluorescent brightening and resin 5 treatment are simultaneously effected in the same bath. The former method has drawbacks in that it requires two steps and inthat the brightness attained by the fluorescent brightening may deteriorate during the subsequent resintreatment. The latter method offers the ad vantage that both fluorescent brightening and resin treatment. can be effected in a single step.

Known methods wherein fluorescent brightening and resin treatment are simultaneously appliedto cellulosic fibers employ synthetic resin precondensates such as SO Na monoor dimethylol urea resin or dimethylol alkylene urea resin and catalysts such as magnesium chloride or zinc sulfate in an acidic dipping bath.

When prior art fluorescent brighteners (for example the compound of formula (A) above) are added to the dipping bath, they often form precipitates in the bath due to the low pH and thus become incapable of imparting the desired fluorescent brightness to the material being treated. The formation of precipitates also often adversely affects the resin treatment itself. In contrast, the compounds of the present invention have excellent acid resistance and do not readily precipitate even at a pH of 3 or lower. They are also compatible with the aforementioned precondensates and catalysts.

In the resin treatment of cellulosic materials, the resin bath is either rendered strongly acidic or a catalyst of the zinc nitrate type is employed in order to improve the effect. When a zinc nitrate catalyst is used, the standard fluorescent brighteners produce a less than satisfactory brightness. The compounds of formula l, in addition to their stability in acidic resintreating compositions, give good brightness when used in resin-treatment process employing a catalyst of the zinc nitrate type and the light resistance of the material 5 treated is not reduced by their use.

ment is not critical. Suitable precondensates, in addition to those already mentioned include, dimethylol urea resin, polymethylol melamine, dimethylol glyoxal monoureido resin, dimethylol triazone resin and derivatives thereof. These may be used either alone or as mixtures of two or more such compounds.

Catalysts with may be used in resin treatment employing the compounds of this invention in addition to those catalysts of zinc nitrate type mentioned above, include inorganic salts such as magnesium nitrate and ammonium sulfate and organic amine salts such as monoethanolamine hydrochloride and 2-amino-2- methylpropanol hydrochloride. These may also be used either alone or as mixtures of two or more such catalysts.

iii. Application to polyamide fibers All of the compounds represented by formula l have cxcellentacid resistance. Consequently, little precipitation takes place, and the fluorescence and dyeability of these compounds is maintained at low pHs. Thus, the

compounds of this invention will impart excellent brightness and good light resistance to polyamide fibers when applied thereto in an acidic bath at'atemperziture of 60 to lO0C, preferably 90 to 100C.

The procedure for preparing the compounds of this 5 invention and for using same will be described in more detail by means of the following examplesfThe exemc/ c j ILH, I NIH SO Na SO Na N t N so na sogna ustom EXAMPLE 1 compound was assimulated to that of the compound of 20 parts of cyanuric chloride was dissolved in 80 parts of acetone and the solution poured into a mixture of 90 parts of ice and 10 parts of water. To the resulting mixture was added dropwise over approximate one hour a solution of 26.6 parts of aniline-2,5-disulfonic acid and 4.4 parts of caustic soda in about l parts of warm water. During the addition, the reaction mixture was kept at 6 to 7C and at pH 3 to 4 by controlled addition of about 16 percent aqueous caustic soda solution, and stirring continued for about 2 hours. The solution gradually turned to clear pale brown color. At the homogeneous. The resulting coating compositions, after being raised to a temperature of 80C, were each 5 homogeneously applied to one side of a leaf of commercially available high quality paper (with no fluorescence) which was then dried on a drum drier to obtain the brightened paper. The brightness thus attained was estimated by comparison, made visually under sun light passed through a northern window, with that attained by the similar application of the prior art dye-stuff of formula (A). The results areset forth in the following point where the test for the presence of the amine betable came negative, 19.5 parts of sodium 4,4-

diaminostilbene-2,2'-disulfonate dissolved in about 150 parts of warm water was added dropwise over apconccmr-mon proximate one and a half hour. During the addition, the B'ighwmr temperature of the reaction mixture was allowed to rise Compound of Sumdurd Standard standard gradually to to C, the pH thereof kept at pH 4 20 formula (A) to 5 with controlled addition of 3 parts of 15 percent Compound of considerably considerably outstandingly d d h l d d formula (2) increased increased increased aqueous so a as so ution an stirring continue untl brightness brightness brightness the sodium 4,4-diaminostilbene-2,2'-disulfonate was no longer detected. To the resulting reaction mixture were added in one portion 102 parts of phenol and 4.4

parts of caustic soda dissolved in such an amount of AS is Cvideht from the data of the above table high water as to make up to about 50 parts. The resulting sobrightness was obtained with the respective concentralution was slowly heated up to 80C, at which temperation levels of the compound Of form l which ture stirring was continued for 4 hours. After the reacfeet was more prominent with higher concentration tion was complete, activated carbon was added to the 30 levels. This is believed to result from the superiority of resulting reaction mixture and filtered to effect purifithe compound of formula (2) over the prior art comcation. The filtrate was salted out at about 70C with pound (A) with respect to its build up property", its 200 parts (20 percent based on the filtrate) of sodium afinity for the coating agent and for paper and with rechloride. The whole was filtered and the filter cake spect to its acid resistance.

dried to give greenish yellow powder identified as sodium 4,4-bis[2-(2,5-disulfoanilino)-4-phenoxy- 1 ,3,5- EXAMPLE 2 triazinyl-6-amino]-stilbene-2,2'-disulfonate of formula The procedure as in Example 1 was repeated except SO Na SO Na SO;,Na S0,,Na

l lH-(ll (l -NH CH=CH Nn c fiNH-- (2) N N N N s Na c c SO Na O 0 To a 3 aqueous solution of a prepared oxidized that 14 parts of p-chlorophenol was used instead of the starch (which may be oxidized starch or a coating agent 10.2 parts of phenol used in Example 1, giving sodium for use in size press process) was added the compound 4,4-bis[2-(2,5-disulfoanilin)-4-( p-chlorophenoxy)- of formula (2) to give 0.4 0.6 and 0.8 aqueous 1,3,5-triazinyl-6-amino] stilbene 2,2-disulfonate havsolutions of said compound (the optical density of the ing the following formula.

SO Na so Na SO Na SO Na N /N\ NHC% \T|INH CH=CH fi-NH (3] N N N v so Na c (1: so,Na

C] Cl formula (A) used as control) followed by stirring until I When a treatment by the size press process was made with the compound of formula (3) in the same manner as described in Example 1, results identical to those of Example 1 were obtained. i

The procedure described above was repeated using' ochl0r0phenol, p-cresol, pmethoxyp henol, 2,4-

SO Na SO Na SO Na SO Na some! N\ NH I CH i =CH NH-L xylenol, m-chlorophenol, p-ter.-butylphenol and poctyilphenol in place of the phenol giving the compounds offormulas(4), (5), (6), 7), 8), (9) and 10) respectively. All of these compounds, when applied to paper in the same manner as in Example 1, gave results identical to those obtained in Example 1.

SO Na (I: i

SO Na SO;,Na

CH=CH SO Na SO Na (II SO Na 0 SOaNZl Nu SOHN v SO Na v p N N t 1 -NH-fi (|NH CH=(H NH]C| (NH 9 N N t v N N v a A 7 so Na IC so Na (I) 0 .i =r CH,(|CH3 CH; i-r

SO;,Nu S Na SO Na SO -,Na N v N NH-|C/ e NH CH=(H -NHC|/ \|C--N 10 N N N N SO Na SO Na N I7 CN IT E AMP E 3 30 no] and calcium chloride was used instead of the sodium chloride, giving the compounds of formulas I l The procedure as described in Example 1 was folnd (12). r p i y These Compounds, n plowed with the exception that 2,4-diehlor0phenol or plied to paper in the same manneras in Example l, 2-methyl-4-ehl0rophenol was used instead of the pheagain gave results as in Example 1.

/SO;,K $0 K SO K SO 1K NH -H/ \(IJNH ICH=CH NHH/ Cl-NH (H) N N N N 50 K C S0,,K

Cl Cl Cl C] $0 K NHICI (IL-NH 12) N N $0 K c SO K EXAMPLE 4 Method afThe brightness attained by application of a solution containin g 3% oxidized starch, l.5 p e'rcent' commercially available'siz ing agent HAMACO'AT S manufacturediby e y s foa 'p'erc'iit compound of formula Twas visually estimated.

Method bf Brightness attained with asolution containing 3% oxidized starch and 0.4 percent compound of formula '2 was estimated. In both a and b the applica"- tion conditions were the same as in Example 1 and the estimation of brightness was 1) made visually and 2) measured using a xenon standard white light source to determine the difference (indicated in table as A Z value) in Z values between the untrea ted and treated fabrics. For purposes of comparison prior art compound B was also applied as in procedures ,4 and bvd e scribed above. Theyresults are set forth in the following creased brightness formula (2) The foregoing shows that when used in combination 7 with a surface sizing agent, the prior art dye-stuff pro duced slightly decreased brightness while the compound of formula 2 gave considerably increased bright n Example 4 was repeated substituting the compounds offormulas 3,4, 5,6, 7, 8,9,10,1land l2forthecorn- M with water then added to give a solids content 40 percent, to give the stated concentrations of said compound and brightener (the optical density of the compound was assimilated to that of the brightener). The compositions thus obtained were stirred until homogeneous and then each was separately applied at normal temperature (20C) to one side of a leaf of high quality paper (without fluorescence) at a coverage of l2 g/m and immediately dried at 909C. The resulting brightness was'thenestimated in the manner set forth in Example l. The results are set forth in the following table.

Concentration of brightener (based on mineral pigment) I As is evident from the foregoing data, in comparison to the prior art brightener, the compound of formula (2)produced, at lower concentration, approximately equal brightness, while at higher concentrations it produced increasingly higher brightness levels.

As far as the prior art brightener is concerned, brightness was highest at 2 percent concentration and a quenching phenomenon was apparently observed at 4.0 percent concentration.

A Brightness obtained by treatment with compound (2) increased almost proportionally with increasing concentration and reached its maximum at about 6 percent concentration.

pound of formula 2. The test results were substantially I the same as those obtained with compound 2 in example 4.

EXAMPLE 6 i Mineral pigment Clay 90 parts Titanium oxide 10 parts a v 1. Casein lO 7: (based on mineral pigment) Latex l0 7:: 7 (do;)

styrene-butadiene copo lymer) ,7 Dispersing agent 0.3 7:-

(Sodium pyrophosphate) EXAMPLE 7 Treatment was repeated as in Example 6 using the compounds of formulas 3, 4, 5, 6, 7, 8, 9, 10, 11 and 12-. in place of the compound of formula 2. The results for each compound were substantially identical to "those obtained for 2 in example 6.

EXAMPLE 8 A0.'4 percent solution of the compound of formula (2) obtained in Example 1 (the optical density of the compound was assimilated to that of the known compound of formula (D))and'a 0.4 percent solution of the prior art compound of formula (D) were each added to a 3 percent solution of oxidized starch and the respective compositions "thus obtained adjusted to three pH levels of 10, 8 and 6 with soda ash or acetic acid. Brightness attained after application of the compositions according to the manner described in Example l was estimated talging the pH 10 solutions as standard. The results are set forth in the following table.

pH l 8 6 ilrightcncr Prior Art Standard Slightly deceased considerably of formula (D) brightness decreased brightness Approximately the same brightness Compound of formula (2) Standard Approximately the same brightness As is apparent from the foregoing data, with the prior art dye-stuff considerable changes in brightness were observed to occur with changes in pH b'ut with the compound of formula 2, brightness remained un changed independent of changes in pH and diminished very slightly with the passage of time.

EXAMPLE 9 The procedure of Example 8 was repeated with the exception that compounds of formulas (3), (4), (5),

resin. In this example two l() -pe rccnt zinc sulfate solutions were prepared. and the prior art compound of formula A was added to one solution and the compound of formula (2) was added to the other solution.

Addition of the prior art compound of formula A almost immediately formed insoluble precipitates. In contrast thereto, no precipitate formed upon addition of compound 2 and a brightened paper with a stable high brightness was obtained by application of the solution containing the compound of formula (2).

EXAMPLE 1 1.

When the compounds of formulas (3), (4), (5), (6), (7]), (8),'(9),(l0), (ll) and (12) were added to zinc sulfate solutions in the manner of example 10, no precipitation occured. Stable brightened papers were obtained by application of each of these solutions.

EXAMPLE 12 The following resin-treating compositions each containing the compound of formula (2) were prepared according to the formulations (a), (b), and (c) shown below.

Method (*All manufactured by Dai Nippon lnk & Chemicals. Inc.)

EXAMPLE 10 As previously noted, it-is often desirable to use fluorescent brighteners in combination with an insolubilizing agent which may be zinc sulfate or an acidic Into each of the above-described resin-treating compositions"(a),'(b) and (c) was dipped a scoured and bleached cotton broad cloth (N0. 40) and the cloth wrung to a weightincrease, based on the weight of the cloth itself, of 65 percent. The cloth was then pre-dried at to C for 10 minutes and subjected to curing at 1509C for 3 minutes. Thereafter, an-aqueous solution containing 0.2 percent soda ash and 0.2 percent MON- OGEN (Trade name; manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) was applied to the cloth at 50 to 60C for 5 minutes after which the cloth was water washed, dehydrated and'dried.

Each of the broad cloths (a), (b) and (c) were of outstandingly brilliant whiteness. Brightness. thus attained was estimated by visually comparingthe same with that obtained by similar application-of the prior art bright ener of formula (E), in sun light passed through a northern window. The brightened cloths were also each tested for light resistance by irradiating the respective brightened cloths for 20 hours using a Xenon lamp fade meter (manufactured by Toyo Rika Co., Ltd.) with the magnitude of fading estimated visually under sun light passed through a northern window. The results are set forth in the following table.

Example 12 was repeated substituting for the compound of formula (2), the compounds of formulas (3 (4), (5), (6), (.7), (8), (9), l0), l l) and l2) respectively. The results for each compound for each of the formulations (a), (b) and (c) were approximately the same as those obtained with the compound of formula 2 in Example 12.

EXAMPLE 14 An. aqueous solution containing 0.5 percent of the compound of formula (2) obtained as in Example 1 (the optical density of the compound was assimilated to that of the known compound (F)) and 2 percent glatreated fabric for 20 hours using a xenon lamp fade meter (manufactured by Toyo Rika Co., Ltd.) with the magnitude of fading again estimated visually under sun light passed through a northern window. The results are set forth in the following table. 1

Prior Art Compound Compound of of formula (F) formula (22) Brightening effect Standard Equal to better Light resistance Standard Better EXAMPLE 15 The procedure of Example 14 was repeated substituting compound (2) with the compounds of formulas (3), (4), (5), (6), (7), (8), (9), (10), (l ll) and (lZl), respectively. Spun nylon treated in the same manner as described in Example 14 with these solutions showed the same brightness and light resistance as that obtained with compound 2 in example 14.

The invention may be em'bodiedin other specific forms without departing from its spirit or essential characteristics. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the claims rather than by the foregoing description, and all changes which come within the meaning and range ofthe equivalents of the claims are therefore intended to be embraced therein.

We claim:

1. A bistriazinylaminostilbene derivative represented by the following formula:

wherein X and Y are the same or different and selected from chlorine atoms, hydrogen atoms, alkyl groups or methoxy groups and M is selected from hydrogen atoms, alkali or alkaline earth metals or ammonium cations or ammonium cations derived from a primary, secondary or tertiary amine.

2. A compound represented by the following formula: t

13. A compound represented by the following formula:

11. A compound represented by the following formula:

m a C N N\ C 0 0 M H /MU M m CIN m C O C SO N 11 SO K SO Nu 14. A compound represented by the following formula:

SO Na H C C N N Vc o a H a N Nu! m. M w w 0 s H f N c o J W m SO NLI a N O S 15. A compound represented by the following mula:

for-

VSO Nu 

1. A BISTRIAZINYLAMINOSTILBENE DERIVATIVE REPRESENTED BY THE FOLLOWING FORMULA:
 2. A compound represented by the following formula:
 3. A compound represented by the following formula:
 4. A compound reepresented by the following formula:
 5. A compound represented by the following formula:
 6. A compound represented by the following formula:
 7. A compound represented by the following formula:
 8. A compound represented by the following formula:
 9. A compound represented by the following formula:
 10. A compound represented by the following formula:
 11. A compound represented by the following formula:
 12. A compound represented by the following formula:
 13. A compound represented by the following formula:
 14. A compound represented by the following formula:
 15. A compound represented by the following formula:
 16. A compound represented by the following formula: 